The oligo- or polymerization of isocyanates, especially to form high molecular weight oligomer mixtures having uretdione (“dimer”), isocyanurate (“trimer”) and/or iminooxadiazinedione structures (“asymmetric trimer”) in the molecular skeleton, collectively called isocyanate modification here, has long been known. The modified polyisocyanates comprising free NCO groups, which optionally may also have been temporarily deactivated with blocking agents, are exceptionally high-quality starting materials for the preparation of a multiplicity of polyurethane plastics and coating compositions.
A series of industrial methods for isocyanate modification have been established in which the isocyanate to be modified, usually a diisocyanate, is generally reacted by addition of catalysts and these are then rendered inactive (deactivated) by suitable measures, when the desired degree of conversion of the isocyanate to be modified has been reached, and the polyisocyanate obtained is generally separated from unreacted monomer. A summary of these methods from the prior art can be found in H. J. Laas et al., J Prakt. Chem. 1994, 336, 185 ff.
Useful modification catalysts have been found to be neutral bases and compounds of ionic composition. The latter can usually be used in a very small amount and lead extremely rapidly to the desired result. In the case of the neutral bases, depending on the monomer to be converted and the neutral base used, this is not always true, but it is virtually impossible to infer structure-effect or -activity relationships (cf. Chem. Eur. J. 2009, 15, 5200-5202).
The option of using tetraorganylammonium or -phosphonium as cation to the anion which is catalytically active toward isocyanates, such as hydroxide, alkanoate, alkoxylate, etc., is common knowledge, although generally not explicitly emphasized as being particularly preferred; cf. H. J. Laas et al., J Prakt. Chem. 1994, 336, 185 ff.
Additionally known is the use of fluorides and hydrogenpolyfluorides, the latter being stable adducts of HF with compounds containing fluoride ions, optionally also in the form of their ammonium or phosphonium salts, for the isocyanate modification, from documents including EP 962 455 A1, EP 962 454 A1, EP 896 009 A1, EP 798 299 A1, EP 447 074 A1, EP 379 914 A1, EP 339 396 A1, EP 315 692 A1, EP 295 926 A1 and EP 235 388 A1.
However, the tetraorganylammonium and -phosphonium (hydrogenpoly)fluorides of the prior art, in the performance of the modification reaction, often have the disadvantage that, when they are used, the reaction can sometimes be maintained only with continuous metered addition of catalyst, meaning that the breakdown of the catalyst in the isocyanate medium proceeds unacceptably quickly for technical purposes compared to the modification reaction.
An additional factor is that, when tetraorganylammonium (hydrogen)polyfluorides are used, an atypical reaction profile is sometimes observed, which leads to products having a much lower iminooxadiazinedione group content than in the case of a regular heat production rate profile (cf. EP 962 455 A1). According to the teaching of EP 962 455 A1, this disadvantage was eliminated by the use of phosphonium salts, but the latter—especially at relatively high reaction temperatures—have the unacceptably high tendency to decomposition mentioned further up, and the decomposition products can have an adverse effect on process and product stability.
EP 2 415 795 A1 describes very stable tetraorganylphosphonium (hydrogenpoly)fluorides that do not have these disadvantages, but they are not commercially available and are preparable only with difficulty.